For optimum treatment of the material in the apparatus it is necessary to discharge the treated material from the apparatus against a controlled counteracting pressure. This problem is solved according to Swedish Pat. No. 314,288 by providing a variable outlet opening between a stationary end-portion of the apparatus and a housing portion shiftable in the longitudinally direction of the screws and biased against the stationary end-portion with an adjustable pressure.
In the construction of a double screw refiner according to British Pat. No. 1,229,894 the housing is stationary and discharge is performed through apertures around the outlet end-portions of the shafts carrying the screw threads. Practical experience has shown that controlled discharge through a variable outlet opening is a factor of great importance for the refining result obtained. The solution of this problem proposed in Swedish Pat. No. 314 288 however, does not appear to be ideal for the following reasons.